This invention relates generally to the field of microwave antennas for transmitting and receiving signals in the spectrum of microwave electromagnetic radiation. More particularly, the invention relates to an offset feed horn for propagating energy toward and receiving energy from a microwave reflector antenna for use especially in satellite-ground communication systems.
Such communication systems have come into widespread use for a variety of military and commercial communications purposes, including the transmission of commercial television signals over long distances, the transmission of telemetry data from space stations and the surveillance and reporting of events on Earth.
With the growth in use of such communication systems has come a corresponding increase in the difficulty of avoiding interference between simultaneous transmissions. Since the number of frequency channels is limited, the need to impose very strict limits on the transmitted beam width has required increasing sophistication in the design of the antennas used.
With improved control of the width and shape of the beams employed the simultaneous use of a single channel by several different communication systems transmitting between geographically separate points becomes possible. In order for such simultaneous use to be practical, however, the antenna designs must severely limit the quantity of microwave radiation which is directed into sidelobes propagating along axes which diverge from the main propagation axis.
The geometries of antenna designs in use for such communication systems are generally modeled after reflecting optical telescope designs such as the Newtonian, Gregorian and Cassegrainian systems. Consequently, most employ single or multiple-arrayed microwave feed horns as the source (in transmission mode) of microwave radiations. The energy propagating from this source is then focused into a more-or-less diverging beam directed toward the receiver or the receiving zone on Earth in the case of satellite-to-ground communications.
By careful optical design, the size and shape of the beam and the signal intensity profile across the extent of the beam can be quite well controlled. The same design procedure can also be employed to control the size and location of sidelobe radiation resulting from residual aberrations in the design of the optical system.
However, an important source of sidelobe radiation derives from spillover within the antenna system itself. Such spillover occurs because the microwave feed horn fails to direct all of its radiation along axes which will intercept the large main reflective antenna. Since spillover radiation avoids the reflective antenna, it cannot be focussed into the main propagating beam and becomes instead a source of unwanted interfering radiation in geographical areas outside the intended transmission zone.
Consequently, an important source of unwanted interfering or sidelobe radiation present in existing antenna designs could be eliminated by preventing the propagation of radiation from the feed horn assembly to the main reflective antenna along axes which do not intersect the surface of the reflective antenna.